1. Field of the Invention
This invention relates to the processing of electrical signals and in particular it concerns novel methods and apparatus for utilizing digital signal processing in electronic theft detection.
2. Description of the Prior Art
U.S. Pat. No. 4,623,877 to Pierre F. Buckens and assigned to the assignee of the present invention discloses and claims methods and apparatus for detecting the unauthorized taking of objects from a protected area, such as a store. Articles taken from the store must pass through an interrogation zone into which electromagnetic interrogation energy is continuously radiated. If, while an article is brought through the interrogation zone, it has an active target mounted thereon, the target will respond to the electromagnetic interrogation energy in the zone and will produce disturbances of that energy in the form of pulses having unique characteristics. These pulses are detected by a receiver at the interrogation zone.
The Buckens invention makes use of signal processing to ascertain the average signal level in the interrogation zone at different portions of each interrogation cycle and to adjust the detection threshold level according to that level so that targets may be detected in the presence of other objects which may also produce interfering signals.